1. Field of the Invention
The present invention relates to a method of sample valuation in which an intensity-modulated excitation light beam is illuminated periodically on to a sample and thermal expansion vibration emerging on the surface of the sample is measured thereby to evaluate the sample for the inspection of defects or the like.
2. Description of the Prior Art
When a light beam which is modulated periodically in terms of the intensity (exciting beam) is illuminated to a sample, it heats up by absorbing the light and develops thermal expansion. Due to the periodical intensity modulation of the illuminated light, the sample undergoes periodic variation of temperature, causing thermal expansion vibration to occur. This thermal response is measured, as it is known to be the technique of measuring the photothermal displacement, thereby valuating the sample.
FIG. 5 shows in brief the arrangement of a prior art apparatus A01 for measuring the thermal expansion vibration of a sample based on the Michelson's laser interference method (described in Miranda, APPLIED OPTICS, Vol. 22, No. 18, p. 2882, (1983)). In the figure, reference numeral 61 denotes a sample under test, 62 is an excitation light source and 63 is a chopper which renders intensity modulation to a light beam emitted by the light source 62, with the modulated beam being illuminated to the sample 61 so that the irradiated sample 61 develops thermal expansion vibration.
For the measurement of this thermal expansion vibration (photothermal displacement) based on the scheme of laser interference, a light beam from a measuring laser source 64 is split by a half mirror 65 into a light beam directed to the thermal expansion measuring position of the sample and a light beam directed to a fixed mirror 66. The reflected lights from the sample 61 and the mirror 66 interfere with each other, and the resulting interference light is received by an opto-electric transducer 67, which produces an electrical output E expressed as follows. EQU E=C.sub.1 +C.sub.2 .multidot.cos(P(t)+.phi.) (1')
where C.sub.1, C.sub.2 and .phi. are constants dependent on the structure of the interferometer, opto-electric conversion factor, etc., and P(t) is the phase variation attributable to the displacement of the sample surface due to the thermal expansion vibration caused by the illumination of the exciting beam. A signal processing circuit 68 is used to measure the thermal expansion vibration, and the characteristic of thermal elasticity of the sample is evaluated.
FIG. 6 shows in brief the arrangement of part of a prior art apparatus A02 which is based on the scheme of using the exciting beam also for the measuring beam (Japanese Patent Application No.5-172948) and was developed by the inventors of the present invention.
In the figure, an exciting beam from an excitation light source 72 is rendered intensity modulation by an acoustic-optic modulator 73, and a resulting diffracted and non-diffracted light beams are illuminated to different positions of a sample 71 by way of a beam splitter 74. The reflected lights from the irradiation positions go back through the beam splitter 74 and interfere with each other in an interferometer made up of mirrors M and a beam splitter 77. The resulting interference light is received by an opto-electric transducer 78, which produces an electrical output E expressed as follows. EQU E=C.sub.3 +C.sub.4 .multidot.A(t).multidot.cos{2.pi.Fbt+P(t)+.phi.}(2')
where C.sub.3, C.sub.4 and .phi. are constants dependent on the structure of the interferometer, opto-electric conversion factor, etc., and A(t) is a factor dependent on the degree of intensity variation produced by the acoustic-optic modulator 73, Fb is the carrier wave frequency of the acoustic-optic modulator 73, and P(t) is the phase variation attributable to the photothermal displacement of the sample caused by the illumination of exciting beam. A signal processing circuit 79 is used to measure the thermal expansion vibration, and the characteristic of thermal elasticity of the sample is evaluated.
In this prior art apparatus A02, the diffracted and non-diffracted light beams are rendered intensity modulation in opposite phase relationship, causing the photothermal displacement pertinent to the reflected lights from the sample 71 to produce a phase difference twice that of the prior art apparatus A01, and the accuracy of measuring the photothermal displacement is enhanced.
FIG. 7 shows in brief the arrangement of a prior art apparatus A03 which is based on the scheme of using the exciting beam also for the measurement (Japanese patent publication JP-A-3-269346) and was developed by the inventors of the present invention prior to the above-mentioned apparatus A02.
In the figure, an exciting beam from an excitation light source 81 is rendered intensity modulation by an acoustic-optic modulator 82. The modulated beam is fed to a frequency shifter 83, by which beams 1 and 2 that are orthogonal to each other and have a frequency difference of Fb are produced. These beams are separated by a beam splitter 84. The beam 1 is illuminated on to a sample 85 through a lens 86, and the beam 2 is directed to a mirror 87 to become a reference light beam.
The reflected light from the sample 85 and the reference beam interfere with each other by being fed through the beam splitter 84 and a polarizing plate 88, and the resulting interference light is received by an opto-electric transducer 89. The transducer 89 produces an output E, which includes the term of phase variation P(t) attributable to the photothermal displacement of the sample caused by the illumination of exciting beam, as expressed by the above equation 2'. A signal processing circuit 90 is used to measure the thermal expansion vibration, and the characteristic of thermal elasticity of the sample is evaluated.
However, the foregoing prior art sample valuation methods based on the measurement of photothermal displacement have the following problems.
(1) The prior art apparatus A01 necessitates to direct the measuring beam to the irradiation position of the exciting beam on the sample. The misalignment of these light beams creates the fluctuation of measured value of the photothermal displacement, and it cannot be measured stably and accurately. When the photothermal displacement is small, the phase variation detected based on the interference measurement is also small, resulting in a degraded accuracy of measurement.
(2) The prior art apparatus A02 and A03 use the exciting beam also for the measuring beam, and accordingly the misalignment of two light beams in the case of the apparatus A01 is dissolved. The apparatus A02 can have the enhanced measuring accuracy as mentioned previously even if the photothermal displacement is small. However, any of these apparatus A02 and A03 creates a phase variation in the electrical signal due to a phase variation in the intensity-modulated beam of the acoustic-optic modulator and a signal level variation of the opto-electric transducer or amplifier. This phase variation will become a noise component superimposed on the phase variation attributable to the photothermal displacement. On this account, both apparatus need to implement intricate signal processings for the noise compensation in order to perform the stable measurement of photothermal displacement.